1. Field of the Invention
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2006-168476, filed on Jun. 19, 2006, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a mobile communications system and, more particularly, to techniques for measuring the channel quality of each mobile station, pilot-resource allocating techniques, and a base station using these techniques.
2. Description of the Related Art
A base station in a mobile communications system performs resource scheduling to enhance the efficiency of radio resources. The scheduling methods can be classified into two broad types; one is channel-independent scheduling by which a resource is allocated periodically, for example by round-robin, irrespective of channel quality; the other one is channel-dependent scheduling by which, taking channel quality into account, a resource is allocated to a mobile station with good channel quality. Since the channel-dependent scheduling has great multiuser diversity effect in comparison with the channel-independent scheduling, it is reported that the channel-dependent scheduling can provide a greater throughput (see Jalali, A., Padovani, R., and Pankaj, R., “Data throughput of CDMA-HDR a high efficiency-high data rate personal communication wireless system,” Proceedings of IEEE VTC2000-Spring, May 2000, pp. 1854-1858). Additionally, for the channel-dependent scheduling, a maximum CIR (Carrier-to-Interference power Ratio) method, proportional fairness method, and the like are proposed.
To perform the channel-dependent scheduling, a base station needs to measure the quality of a channel (hereinafter, referred to as CQI: Channel Quality Indicator) between the base station and each mobile station (UE: User Equipment). In the channel-dependent scheduling, a resource is allocated to a mobile station with the best CQI. Therefore, CQI measurement needs to be done over a range, as a scheduling band, including not only a frequency band (frequency block) where data transmission is being performed, but also frequency blocks having the possibility for data transmission. For example, see 3GPP R1-050701 (NTT DoCoMo et al., “Channel-Dependent Scheduling Method for Single-Carrier FDMA Radio Access in Evolved UTRA Uplink,” Aug. 29-Sep. 2, 2005)). it is proposed that CQI measurement is made by utilizing a pilot signal (also referred to as reference signal) multiplexed on an uplink from each mobile station to the base station (see 3GPP R1-060925 (Texas Instruments, “Comparison of Proposed Uplink Pilot Structures for SC-OFDMA,” March 2006)). In other words, a pilot signal for demodulating an uplink data signal and a control signals, is also used to measure CQI.
According to such a method, the base station measures CQI based on pilot signals from the mobile stations and allocates a data resource to a mobile station with the best CQI. If the base station has downlink data to transmit, the base station allocates a control resource for transmitting a control signal for the downlink data. If an allocation of the data resource or control resource takes place, CQI measurement is carried out by utilizing the pilot signal for demodulating the corresponding data or control signal. On the other hand, when a mobile station receives control information regarding resources from the base station, the mobile station transmits a data signal, control signal, and pilot signal to the base station in accordance with the received resource information.
However, according to the above-described method, the pilot resource cannot be efficiently allocated, as will be described below. Hereinafter, this will be described with reference to FIGS. 1A, 1B and 2A.
FIG. 1A is a block diagram showing an example of a mobile communications system, and FIG. 1B is a table showing the transmission states of mobile stations in a frequency block. Here, it is assumed that N mobile stations (UEs) are connected to a single base station 100.
In this case, the transmission states of the mobile stations in a frequency block can be classified into four groups a to d, as shown in FIG. 1B. In a certain frequency block, mobile stations belonging to the groups a to c are transmitting any one or both of an uplink data signal and a control signal. Therefore, CQI measurement can be performed for these mobile stations by using the pilot signals for demodulating these signals. However, a mobile station belonging to the group d has sent an uplink-data transmission request (hereinafter, also referred to as resource request) to the base station but is still waiting for transmission of an uplink data signal, even without transmitting an uplink control signal yet. To perform CQI measurement for the mobile stations including such a mobile station belonging to the group d in a scheduling band, according to a hitherto-used technique, pilot signals are transmitted across a bandwidth wider than the bandwidth where all the mobile stations are actually transmitting data signals and control signals, which will be described below more specifically.
FIG. 2A is a diagram schematically showing the allocation of a resource for CQI measurement according to a hitherto-used technique. The numerals after “UE” in FIG. 2A represent the mobile station numbers (in the drawings, a mobile station is expressed as UE where appropriate). In this example, a control signal, pilot signal, and data signal are multiplexed in time division scheme (TDM: time division multiplexing), and the resources to be allocated for these signals are referred to as a control resource, pilot resource, and data resource, respectively. Moreover, in the control resource, control signals from a plurality of mobile stations are multiplexed by means of distributed FDM (frequency division multiplexing) using the entire scheduling band, and in the pilot resource, pilot signals from the plurality of mobile stations are multiplexed in code division scheme (CDM: code division multiplexing). A control signal in this example is an uplink control signal regarding a downlink data signal (called data non-associated control signaling) and contains downlink CQI, ACK/NACK indicating whether or not downlink packets have been correctly received, or the like.
Although the transmission amount of a control signal is smaller than that of a data signal, the control signal needs to be transmitted periodically. As shown in FIG. 2A, in resource allocation according to the hitherto-used technique, every mobile station transmits a pilot signal using the entire scheduling band in which CQI is measured. Accordingly, as to mobile stations belonging to the groups a to c in FIG. 1B, each pilot signal is used by the base station for two purposes: modulation of a data signal and/or control signal, and CQI measurement. However, as to those belonging to the group d, each pilot signal is similarly transmitted only for CQI measurement over the scheduling band. That is, a pilot resource for no particular use is allocated to these mobile stations. In addition, a pilot resource for no particular use is sometimes allocated similarly even to a mobile station belonging to any one of the groups a to c, because the same entire band is used to transmit pilot signals. As a result, resource allocation techniques as shown in FIG. 2A have had the problem that the overhead is large and the pilot resource cannot be efficiently allocated.
In addition, studies have been made to achieve pilot signal orthogonality among mobile stations, with it being known that there is a limit to the number of mobile stations which can make orthogonal signals (see 3GPP R1-060319 (NTT DoCoMo et al., “Orthogonal Pilot Channel Structure for E-UTRA Uplink,” February 2006)). Due to this limit, the problem also arises that the number of mobile stations for which the base station performs CQI measurement becomes smaller when a pilot signal is used both for demodulation and CQI measurement.